Magnetorheological (MR) fluids are responsive to magnetic fields and contain a field polarizable particle component and a liquid carrier component. MR fluids are useful in a variety of mechanical applications including, but not limited to, shock absorbers, controllable suspension systems, vibration dampeners, and electronically controllable force/torque transfer devices.
The particle component of MR fluids typically includes micron-sized magnetic-responsive particles. In the presence of a magnetic field, the magnetic-responsive particles become polarized and are organized into chains or particle fibrils which increase the apparent viscosity (flow resistance) of the fluid, resulting in the development of a solid mass having a yield stress that must be exceeded to induce onset of flow of the MR fluid. The particles return to an unorganized state when the magnetic field is removed, which lowers the viscosity of the fluid.
Oxidation of ferromagnetic particles is particularly pronounced at elevated temperatures. This makes the use of MR fluids in high temperature applications such as automotive fan and transmission clutches particularly problematic.
Thus it would be desirable to provide an MR fluid containing iron particles that are resistant to oxidation. It would also be desirable to provide particles useful in MR fluids that are oxidation resistant but exhibit significant magnetization response.